There was indeed such a rule in the sect's sage realm. However, as the saying goes, it's better to make enemies than to make them. As long as it wasn't a hatred of killing one's father or stealing one's wife, generally, no one would come to find trouble during the opening ceremony of another sect. This was no better than slapping the face in front of all the guests. If it wasn't a mortal enmity, it had become a mortal enmity.
And those who wanted to establish their own sects, of course, would not be willing to encounter such trouble on their own day of founding. Before that, they would settle all their grudges properly.
Therefore, there was such a rule, but there were very few people who came to seek revenge at the sect opening ceremony, so many people ignored the rule of the sage realm.
